The present invention relates to a conjugate fiber nonwoven fabric not only excellent in softness, but also high in strength, and to a nonwoven fabric for use in sanitary materials which utilize the above conjugate fiber nonwoven fabric.
Spunbonded nonwoven fabrics, which have been used in a wide variety of applications in recent years, offer the advantages of being excellent in tensile strength and high in productivity over short fiber nonwoven fabrics produced by the carding or the melt-blowing process. On the other hand, compared with the short fiber nonwoven fabrics, they are poor in softness, therefore, they are less adequate for applications where they directly touch a person""s skin, for example, for the application to surface material for sanitary goods. However, for applications to disposables, spunbonded fabrics are suitable due to their high productivity; thus there have been adopted various techniques for producing spunbonded fabrics more excellent in softness.
For example, there has been made a proposal to provide properly spaced binding zones for binding fibers, which are to be formed into a nonwoven fabric, so that the fibers can fusion bond together exclusively within the binding zones, whereby regions where fibers do not bind with each other can be created.
The nonwoven fabrics, however, have not exhibited an adequate softness with this technique alone.
A polyethylene nonwoven fabric, which resin fibers are formed of polyethylene, is known for its softness and good touch (Japanese Patent Laid-Open No. 60-209010). Polyethylene fibers are, however, difficult to spin, and hence difficult to allow to have a fine denier. And a nonwoven fabric formed of polyethylene fibers easily melts when subjected to heat/pressure treatment with a calender roll, and what is even worse, it easily winds itself around the roll due to low strength of the fibers. Measures have been taken against the above problems in which the treatment temperature is decreased; however, in such a case, thermal adhesion is apt to be insufficient, which leads to another problem of being unable to obtain nonwoven fabric with sufficient strength and fastness to rubbing. In actuality, a polyethylene nonwoven fabric is inferior to a polypropylene nonwoven fabric in strength.
In order to solve the above problems, there have been proposed techniques of utilizing a core-sheath-type conjugate fiber using a resin of polypropylene, polyester, etc., as a core, and polyethylene as a sheath (Japanese Patent Publication No. 55-483, Japanese Patent Laid-Open No. 2-182960 and Japanese Patent Laid-Open No. 5-263353).
However, the currently proposed nonwoven fabrics, which are formed of core-sheath-type conjugate fibers as described above, have not had both softness and strength adequate to be used as sanitary materials. Specifically, when increasing the amount of polyethylene as a constituent of sheath, the softness of the nonwoven fabric is enhanced, but its strength is not allowed to be sufficient, as a result of which it is likely to fracture during the process. On the other hand, when increasing the constituent of core, the nonwoven fabric is allowed to have sufficient strength, but is poor in softness and its quality, as a material for sanitary goods, decreases. Thus it has been difficult to obtain a nonwoven fabric having both of the above performances on a satisfactory level.
Accordingly, the object of the present invention is to solve the aforementioned problems the background arts have, in particular, to provide a conjugate fiber nonwoven fabric with excellent softness and touch as well as with sufficient strength.
In order to achieve the above object, the present inventors provide a conjugate fiber nonwoven fabric which comprises conjugate fibers, preferably core-sheath-type or side-by-side-type conjugate fibers, composed of a polyethylene-based resin (A) having a higher melting point in the range of 120 to 135xc2x0 C. and a lower melting point in the range of 90 to 125xc2x0 C., which is lower than the above higher melting point at least by 5xc2x0 C., and a high-melting point resin (B) whose melting point is higher than that of the above polyethylene-based resin (A) by 10xc2x0 C. or more, the component ratio by weight of the polyethylene-based resin (A) to the high-melting point resin (B) being in the range of 50/50 to 10/90, and the polyethylene-based resin (A) forming at least part of the surface of the fiber longitudinally continuously.
In a preferred embodiment of the present invention, desirably the above polyethylene-based resin (A) comprises an ethylene-based polymer having a higher melting point in the range of 120 to 135xc2x0 C. and a lower melting point in the range of 90 to 125xc2x0 C. which is lower than the above higher melting point at least by 50xc2x0 C.
Desirably the above polyethylene-based resin (A) comprises an ethylene-based polymer (A-1) having a higher melting point in the range of 120 to 135xc2x0 C. and an ethylene-based polymer (A-2) having a lower melting point in the range of 90 to 125xc2x0 C. which is lower than the above higher melting point at least by 5xc2x0 C.
In this case, preferably the weight ratio [(A-1)/(A-2)] of the ethylene-based polymer (A-1) to the ethylene-based polymer (A-2), both contained in the polyethylene-based resin (A), is in the range of 75/25 to 30/70.
Preferably the density of the ethylene-based polymer (A-1) is in the range of 0.930 to 0.970 g/cm3 and that of the ethylene-based polymer (A-2) is in the range of 0.860 to 0.930 g/cm3.
In the present invention, suitably the above polyethylene-based resin (A) has a molecular weight distribution (Mw/Mn) in the range of 1.5 to 4.0 when measuring by the gel permeation chromatography (GPC).
Suitably the high-melting point resin (B) is propylene-based polymer having a molecular weight distribution (Mw/Mn) in the range of 2.0 to 4.0 when measuring by the GPC.
Preferably the propylene-based polymer is a propylene-ethylene copolymer with a melt flow rate in the range of 20 to 100 g/10 min (measured at a load of 2.16 kg and at 230xc2x0 C. in accordance with ASTM D1238) and an ethylene-derived structural unit content in the range of 0.1 to 5.0 mol %.
The present invention also provides a nonwoven fabric used as sanitary materials which is formed as a laminate of the above conjugate fiber nonwoven fabric and a melt-blown nonwoven fabric.